As examples of a photoelectronic device using an organic semiconductor material as a functional organic material, a light-emitting element, solar battery, and the like can be nominated. These are devices utilizing an electrical property (carrier transporting property) or an optical property (light absorption or light-emitting property) of the organic semiconductor material. Among others, a light-emitting element has achieved remarkable development.
A light-emitting element comprises a pair of electrodes (anode and cathode) and a layer containing a light-emitting material interposed between the pair of electrodes. The emission mechanism is as follows. Upon applying voltage through the pair of electrodes, holes injected from the anode and electrons injected from the cathode are recombined with each other at an emission center within the layer containing a light-emitting material to lead the formation of molecular excitons, and the molecular excitons return to the ground state while radiating energy as light. There are two excited states possible from the light-emitting material, a singlet state and a triplet state. It is considered that light emission can be obtained through both the singlet state and the triplet state.
Lately, the reduction of driving voltage is successful (see Unexamined Patent Publication No. 10-270171) by forming an electron injecting layer made from an organic compound doped with metal having a low work function (metal having an electron donor property) such as an alkali metal, an alkaline earth metal, or a rare earth metal to lower an energy barrier in injecting electrons from a cathode to an organic compound. According to this technique, driving voltage can be reduced despite of using stabilized metal such as Al for forming the cathode.
By using the application of the technique, the control of an emission spectrum of a light-emitting element is successful (see Unexamined Patent Publication No. 2001-102175). In the Unexamined Patent Publication No. 2001-102175, an electron injecting layer made from an organic compound doped with metal having electron donor property for the organic compound is provided. The Unexamined Patent Publication has disclosed that the thickness of the electron injecting layer is increased to vary the optical path between a cathode and a light-emitting layer, so that an emission spectrum emitted to outside can be controlled due to the effect of interference of light.
According to the Unexamined Patent Publication No. 2001-102175, the increase of driving voltage is small by adopting the foregoing electron injecting layer despite of increasing the thickness of the electron injecting layer in order to control an emission spectrum. However, in fact, unless a peculiar organic compound serving as a ligand such as bathocuproin (BCP) is used, driving voltage is drastically increased.
Therefore, the technique with respect to an electron injecting layer disclosed in the Unexamined Patent Publications Nos. 10-270171 and 2001-102175 has a problem that even if the thickness of the electron injecting layer is increased to improve a manufacturing yield, or to control an emission spectrum so that color purity is improved, power consumption is increased unless an organic compound serving as a ligand is selected to use.
A principle of operation of a light-emitting element disclosed in the Unexamined Patent Publications Nos. 10-270171 and 2001-102175 is explained hereinafter with reference to FIG. 2.
FIG. 2 illustrates a basic configuration of the conventional light-emitting element using an electron injecting layer as disclosed in the Unexamined Patent Publications Nos. 10-270171 and 2001-102175.
In the conventional light-emitting element (FIG. 2), holes injected from an anode 201 and electrons injected from a cathode 204 are recombined upon being applied with forward bias to emit light within a layer containing a light-emitting material 202. In this instance, an electron injecting layer 203 is made from an organic compound doped with metal having a high electron donor property for the organic compound (alkali metal or alkali earth metal).
The electron injecting layer 203 serves for flowing electrons to inject them to the layer containing a light-emitting material 202. However, since electron mobility of an organic compound is two orders of magnitude less than hole mobility of that, driving voltage is increased if the electron injecting layer is formed to have a thickness comparable in a wavelength of visible light (on the order of submicron) in order, for example, to control an emission spectrum.